1. Field of the Invention
The present invention relates to a control for pumps in a vehicle application. More particularly, it relates to an improved control for controlling a vacuum pump for supply of a negative pressure to a force multiplier or a vacuum tank generally included in a braking apparatus for vehicles.
2. Description of the Prior Art
FIG. 1 is an electric circuit diagram showing a conventional control technique for pumps used in vehicles. Referring to FIG. 1, the conventional control will be discussed with respect to its schematic structure. The primary function of a vacuum pump 1 is to supply a negative pressure to a pressure-operating unit 10. A representative pressure-operating unit 10 comprises a force multiplier or a vacuum tank generally included in a vehicle braking apparatus. As is well known in the art, the force multiplier functions to augment the force provided when the driver steps on the brake. The vacuum tank is used to store a vacuum pressure set up by the vacuum pump 1. The vacuum pump 1 is driven by a DC motor 2 which, combined with the vacuum pump 1, forms a vehicle pumping device. The DC motor 2 is controlled by a control circuit 3 which includes a power transistor 31 connected in series with the DC motor 2, a driver transistor 32 controlling operation of the power transistor 31, base resistors 33 and 34 for the respective transistors 31 and 32, a surge absorbing diode 35 and a base-emitter resistor 36. The DC motor 2 is fed with a DC voltage from batteries 5 via a power switch 6. A pressure detecting switch 41 is provided in conjunction with a force multiplier or a vacuum tank for detecting a negative pressure generated by the vacuum pump 1 and has a normally closed contact which is open when the negative pressure reaches a predetermined value.
FIG. 2 is a diagram which will give a better understanding of operation of the conventional control as shown in FIG. 1. Operation of the conventional control will be described by reference to FIGS. 1 and 2. Under the condition where the engine (not shown) has not started up and the vehicle is stopped, the negative pressure in the force multiplier or the vacuum tank is generally low and is close to the atmospheric pressure. The contact of the pressure detecting switch 41 is therefore in the closed position. Upon closure of the power switch 6 under these circumstances the driver transistor 32 is fed at its base with a low level signal from the pressure detecting switch 41 and rendered nonconductive. Current flows from the batteries into the base of the power transistors 31 through the power switch 6 and the base resistor 33, rendering the power transistor 31 conductive. In response to conduction of the power transistor 31, the DC voltage from the batteries 5 is supplied to the DC motor 2 which in turn rotates to activate the vacuum pump 1.
It is noted that the pressure in the force multiplier or the vacuum tank decreases gradually and the negative pressure increases. If the negative pressure in the force multiplier or the vacuum tank reaches a first predetermined value, then the normally closed contact of the pressure detecting switch 41 is opened. The driver transistor 32 becomes conductive in response to the base current flowing from the batteries 5 through the power switch 6 and the base resistor 34. As a result, the power transistor 31 becomes non-conductive and the DC motor 2 is no longer supplied with the DC voltage so that the vacuum pump 1 comes to a halt. As the negative pressure in the force multiplier or the vacuum tank decreases and drops to a second predetermined value, the contact of the pressure detecting switch 41 is closed again and the DC motor 2 is energized. The above described sequence of operations is repeated in such a manner as to keep the negative pressure in the force multiplier or the vacuum tank within the range from the first predetermined value to the second.
However, provided that braking is effected intermittently as seen in FIG. 2(a) in the conventional control, the negative pressure for activating the force multiplier demonstrates a sharp decline as plotted with the solid line in FIG. 2(b). Operation of the force multiplier is less powerful and demands application of an even greater force of stepping on the brake. In case of complicated braking operation, the control apparatus has the problem that the DC motor 2 is liable to inch and may enjoy only a very limited length of life.